1. Field of the Invention
The present invention relates to a multi-channel SQUID (superconducting quantum interferometer device) fluxmeter for detecting magnetic flux signals on the basis of pulse signals from a plurality of SQUID flux sensors.
2. Description of the Related Art
High-sensitivity fluxmeter using superconducting quantum interferometer devices (SQUIDs) are employed for measuring small magnetic fields generated by an object under examination, such as a living body. By measuring the distribution of magnetic fields in the brain or the heart, for example, it is possible to estimate the currents which generate them. It is pointed out, from the diagnostic point of view, that the estimation of currents provides very meaningful information and is useful for clarifying the neural activity within a living body. Magnetic fields may be measured with a one-channel SQUID fluxmeter which is adapted to sequentially measure time series data from each part. However, this method takes a long time and the person under examination gets tired. In addition, since the simultaneous measurement of magnetic fields cannot be made in different parts, the currents cannot be estimated with great accuracy. A multi-channel SQUID fluxmeter is thus required, which permits simultaneous measurement of magnetic fields from different portions of the organ under examination.
In a conventional multi-channel SQUID fluxmeter, a processing circuit for converting a detected signal to a flux signal is provided for each one-channel flux sensor. Thus, if, for example, a 100-channel SQUID fluxmeter is to be realized, 100 processing circuits are needed. In analog SQUIDs, a method has also been proposed for simplifying the SQUID system by driving each of the SQUIDs with a different frequency and multiplexing the output lines of the SQUIDs to be sent over a single output line (Furukawa et al., Japanese Journal of Applied Physics, vol. 28, No. 3, Mar. 989, pp L456-L458). This method, however, requires as many phase detector circuits for demodulation as there are channels. Therefore, problems with the conventional device are its increased circuit scale and its increased power consumption.
Some digital SQUIDs are adapted to provide pulse outputs. One type consists of a two-junction quantum interferometer device and is A.C. biased to output a pulse output (refer to Japanese Unexamined Patent Publication No. 63-290979). Another is a D.C. SQUID which applies a voltage to a superconducting comparator or a 1-bit A/D converter to thereby provide a pulse output (D. Drung, Cryogenics, vol. 26, pp 623 -627, 1986).